The structure of this patent application is an improvement of the structure defined within U.S. patent application Ser. No. 476,873, filed June 6, 1974 now U.S. Pat. No. 3,885,365, patented May 27, 1975.
The design of stairways is dependent on several factors and requires complying with minimum dimensional safety standards of the various existing building codes. Example: The rise (abbreviated r) shall not exceed 71/2 inches in commercial buildings or eight inches in houses and the net tread or run (abbreviated t) shall not be less than 10 inches in commercial buildings or nine inches in houses. Some codes require a minimum nosing depth when the run is less than a specified limit. The maximum variations in the rise and run in any one flight is one-quarter of an inch. Thus, the codes attempt to regulate accuracy of construction, but recognize that it is nearly impossible to achieve equal variation of all constructed parts in any one flight. All stairs previously constructed, with the exception of the stair of the aforesaid patent application, exhibit some defect in trying to achieve this accuracy of uniform variation.
Stair design has historically relied on various, and different empirical rules of thumb equivocal building safety codes, and industry standards. The "best" rule was a matter of opinion and the individual designers usually resorted to code values close to the minimum tread and the maximum riser to save stairwell space. Recent scientific studies for the first time based on extensive "user" testing for both comfort (lowest rate of user energy expenditure during ascent) and safety (the fewest user missteps occuring during descent regardless of speed) has disclosed that standardization may now be possible and recommended with regard to riser height and tread depth and their proportions to each other.
Actual testing for comfort has revealed that the rate of user energy expenditure is always higher for higher risers unless these higher risers are combined with a corresponding reduction in the net depth of the treads at which time thhe user comfort approaches the lowest uniform energy rate. Also, stairs with nosings are more accommodating than without nosings because more toe and heel space depth is made available beyond the net tread depth.
Since the average anthoropometric requirements of the user do not change, there is no reason why the use should be unduly subjected to unsafe, uncomfortable and different stairs. The user should not have to shift his gait between the upstairs and the basement in his own house, or when he goes outside, or goes to work in public areas, or encounters industrial stair work situations. Rules can now be derived for comfort or safety or both in equations and pre-scheduled form with confidence.
Presently a rise (r) of 71/2 inches maximum, combined with a run (t) of 10 inches minimum will satisfy seven different published codes and standards. The accompanying dissimilar proportional comfort rules in some of these codes and standards (not not all) are 2r + t = 25 inches; r + t = 17.5 inches; r .times. t =75 inches; 4r + 3t = 60 inches and 2r + 3t = 45 inches which can all be satisfied by r = 71/2 inches and t = 10 inches. Some of these rules waste total run space needlessly while other rules are so blatantly equivocal that most codes grant exceptions to their own rules by making trivial distinctions between public and private uses of stairs.
The convention for constructing stairs is for an architect or other designer to make detailed design drawings, then require by specification that separate, more fabrication drawings be made by the fabricator and returned for the designer's approval. The fabrication drawings should take into account variable field conditions as much as possible, but this is seldom done in practice. Approved fabrication drawings are used for shop fabrication of the stairs. The stair built then becomes a rigid framework for field erection. Unfortunately there are variations in the stairwell space provided for its installation or erection. Only during installation can the errors of stair design in relation to the stairwell space it is suppose to fit be assessed and corrected. Often, several different trades are required to cooperate with each other to complete the installation over a long period of time. It is also common for stair risers or treads to be shortened or extended at the landings just to make the stair fit. Sometimes landings have to be field modified because the rigidly constructed stair is too difficult to correct.
Previous to this invention and the invention of the aforementioned patent application, custom stairs were fabricated from all manner of manufactured stock materials such as rolled steel shapes, bent plates, welded metal plates, and various kinds of fasteners of different sizes. These stock materials had to be reworked extensively by operations such as cutting, piecing, fitting and extensive welding in order to produce a custom design.
Previous to this invention and the invention of the aforesaid patent application, field violations of the code limits were often overlooked or avoided by inspectors because to construct the stair precisely in accordance with the approved drawings was normally just not expected. It is a rare occurrence to have the run and the rise within each stair flight exactly equal or to have one flight match the next flight.